1. Field of the Invention
The present invention relates to an amplitude regulation circuit for regulating an amplitude of a polyphase alternating signal, and a motor driving control apparatus that uses the amplitude regulation circuit.
2. Related Art
Conventionally, brushless direct-current (DC) motors are widely used in head drums of tape recorders and for rotating storage media of disk recorders.
A typical brushless DC motor includes three coils and three Hall elements in pairs, which are arranged in a stator 120° apart in rotation angle. As a rotor rotates, the three Hall elements output three rotor position signals which are out of phase with each other by 120°.
Such a brushless DC motor is driven by a driving control apparatus which supplies currents to the coils according to reference signals that are generated by adding appropriate phase delays (e.g. 30°) to the rotor position signals.
One example of this driving control apparatus is disclosed by Japanese Patent Application Publication H02-188183.
FIG. 12 is a functional block diagram showing a driving control apparatus 9 which represents part of the disclosure of the above document that is relevant to the present invention. In the drawing, Hall elements 91 to 93 and coils 94 to 96 are part of a motor that is driven by the driving control apparatus 9.
The Hall elements 91 to 93 receive power from a power supply 90, and output rotor position signals H1 to H3 respectively. Variable gain amplifiers 21 to 23 respectively amplify rotor position signals H1 to H3 and output signals X1 to X3. Subtraction circuits 31 to 33 respectively calculate difference signals P1 to P3 which each represent a difference between two signals of adjacent phases out of signals X1 to X3. Current driving circuits 41 to 43 respectively supply currents according to difference signals P1 to P3, to the coils 94 to 96.
In this driving control apparatus 9, gains of the variable gain amplifiers 21 to 23 are automatically controlled (automatic gain control (AGC)) so that amplitudes of signals X1 to X3 are kept constant regardless of variations in factors such as Hall element characteristics, temperature, power supply, and the like. AGC makes it possible to stably drive the motor despite variations in these factors.
To do so, an absolute value addition circuit 19 adds together absolute values of signals X1 to X3, and outputs amplitude detection signal Y. A comparator 25 outputs a gain control signal to each of the variable gain amplifiers 21 to 23, based on a comparison between amplitude detection signal Y and a reference voltage generated by a reference voltage generator 26 As a result, the amplitudes of signals X1 to X3 are held constant according to the reference voltage.
FIG. 13 shows waveforms of main signals in the driving control apparatus 9. FIG. 13A shows rotor position signals H1 to H3. Rotor position signals detected by Hall elements have sinusoidal-like waveforms that vary according to a rotating magnetic field. FIG. 13B shows a signal obtained by adding together the absolute values of rotor position signals H1 to H3. This signal has a pulsating waveform (which is not observed in actual circuitry). The amplitudes of rotor position signals H1 to H3 are each limited (by AGC) at peak portions of this pulsating waveform, as a result of which distorted trapezoidal signals X1 to X3 are generated (not illustrated).
FIG. 13C shows difference signals P1 to P3 which each have a complex waveform generated by subtracting one distorted trapezoidal waveform from another.
Thus, the driving control apparatus 9 according to the conventional technique regulates rotor position signals H1 to H3 at constant amplitudes, to thereby drive the motor stably. The driving control apparatus 9, however, cannot drive the motor with low noise and low vibration. Motor noise and vibration pose significant problems especially in devices such as disk devices used for AV (audio/video) equipment and the like.
To drive the motor with low noise and low vibration, coil currents need be smoothly increased and decreased preferably in accordance with pure sinusoidal waveforms, in order to suppress unwanted torque fluctuations. The driving control apparatus 9, however, uses difference signals P1 to P3 generated from signals X1 to X3 which have distorted trapezoidal waveforms. This causes unwanted torque fluctuations.
For example, the amplitudes of signals X1 to X3 may be held constant without distorting the sinusoidal waveforms of rotor position signals H1 to H3, if amplitude detection signal Y is passed through a smoothing capacitor so as to remove a ripple.
However, when a rotation speed of the motor is low such as immediately after starting the motor or immediately before stopping the motor, a ripple frequency is as low as or even lower than 10 Hz. A large smoothing capacitor of 10 μF to 100 μF is needed to obtain a cutoff frequency that is low enough to remove such a ripple. A time required to charge such a large capacitor causes a drop in AGC responsiveness.
A well-known AGC circuit detects output signals of variable gain amplifiers, and controls gains of the variable gain amplifiers according to a control voltage obtained by smoothing the detected signals using a smoothing capacitor. In this case too, a ripple in the control signal when the motor rotation speed is low such as immediately after starting the motor or immediately before stopping the motor causes distortions in outputs of the variable gain amplifiers, with it being impossible to drive a motor stably. A large smoothing capacitor of 10 μF to 100 μF is needed to remove such a ripple at low motor rotation speed. The use of a large smoothing capacitor, however, leads to a drop in responsiveness when the motor rotation speed varies or when the detected signals change. Hence it is still impossible to reduce signal distortions when the motor rotation speed is low, while maintaining high AGC responsiveness.